A power sensor circuit is used to monitor the power requirements of a circuit. In general, such a power sensor circuit may be used with, but not limited to, systems containing an internal battery and powered by an external power supply. For example, if in such a circuit a reduction of power provided by an external power supply is detected, then the battery is selected to provide power to the circuit. When the external power from the external power supply is restored above the predetermined voltage, the battery then ceases to deliver power until the next event occurs. However, for such power sensor circuitry to be useful, the circuit must detect the loss of power from the external source and not consume much power on their own. The present invention relates to a low power consumption power sensor circuit.